1. Field of the Invention
The present invention relates to a fuel cell.
2. Discussion of the Background
A solid polymer fuel cell includes unit cells. Each of the unit cells includes an electrolyte membrane electrode assembly (MEA) and a pair of separators sandwiching the MEA therebetween. The MEA includes an electrolyte membrane, which is a solid polymer ion-exchange membrane, and an anode electrode and a cathode electrode sandwiching the electrolyte membrane therebetween. The solid polymer fuel cell, which usually includes a certain number of stacked unit cells, is used as a fuel cell stack for an automobile.
In the above fuel cell, a fuel gas channel for supplying a fuel gas to the anode electrode is formed in a surface of one of the separators, and an oxidant gas channel for supplying an oxidant gas to the cathode electrode is formed in a surface of the other of the separators. Moreover, a coolant channel for supplying a coolant to an electrode area is formed between the separators of the fuel cells that are adjacent to each other.
As the separator, a metal separator may be used instead of a carbon separator, because a thin separator can be easily made by using a metal. In such a case, a thin metal plate is press forming so as to form channel grooves having a wave shape. By selectively using the channel grooves as a fuel gas channel or an oxidant gas channel, either an anode separator or a cathode separator can be made.
Grooves formed on the back side of the fuel gas channel are superposed on grooves formed on the back side of the oxidant gas channel, and thereby a coolant channel is formed between the anode separator and the cathode separator, which are adjacent to each other.
Japanese Unexamined Patent Application Publication No. 2007-141552, for example, describes a fuel cell stack of this type. FIG. 7 illustrates a unit cell 1 of the fuel cell stack. The unit cell 1 includes a membrane electrode assembly 2 and separators 3 and 4 that sandwich the membrane electrode assembly 2 therebetween.
A fuel gas inlet manifold 5a and an oxidant gas inlet manifold 6a extend through the upper end portion of the unit cell 1 in the stacking direction. A fuel gas outlet manifold 5b and an oxidant gas outlet manifold 6b extend through the lower end portion of the unit cell 1 in the stacking direction. Four cooling water inlet manifolds 7a and four cooling water outlet manifolds 7b are arranged in the vertical direction in lateral end portions of the unit cell 1.
Fuel gas channels 8a having a wave shape are formed in a surface of the separator 3 that faces the membrane electrode assembly 2. The fuel gas channels 8a are connected to the fuel gas inlet manifold 5a and the fuel gas outlet manifold 5b and extend in the longitudinal direction. Oxidant gas channels 9a having a wave shape are formed in a surface of the separator 4 that faces the membrane electrode assembly 2. The oxidant gas channels 9a are connected to the oxidant gas inlet manifold 6a and the oxidant gas outlet manifold 6b and extend in the longitudinal direction.
When the unit cells 1 are stacked on top of each other, a cooling water channel is formed between the separator 3 of one of the unit cells 1 and the separator 4 of an adjacent unit cell 1. The cooling water channel is formed because grooves 8b formed on the back side of the fuel gas channels 8a is superposed on grooves 9b formed on the back side of the oxidant gas channels 9a. The cooling water channel allows a coolant to flow in the lateral direction (horizontal direction) and connects the cooling water inlet manifold 7a to the cooling water outlet manifold 7b. 
In a fuel cell, the flow directions of cooling water, fuel gas, and oxidant gas may be set to be substantially the same. For example, in the unit cell 1, the cooling water outlet manifolds 7b may be interchanged with the cooling water inlet manifolds 7a, and a pair of cooling water inlet manifolds 7a may be formed in the upper part of the unit cell 1 on either side and a pair of cooling water outlet manifolds 7b may be formed in the lower part of the unit cell 1 on either side.
Because the grooves 8b and 9b meander in wave shapes, channels through which cooling water can flow in the horizontal and vertical directions are formed between the grooves 8b and 9b. Thus, a so-called “H-flow” is formed in the sense that the cooling water is introduced through the pair of cooling water inlet manifolds 7a oppositely inward in the lateral direction, the cooling water flows vertically downward, flows outward in the lateral direction, and is discharged from the pair of cooling water outlet manifolds 7b. 
However, in the cooling water channel, after flows of the cooling water join at a part in which the grooves 8b and 9b overlap each other, the flow direction of the cooling water is easily changed. Thus, the cooling water flows diagonally inward with respect to the longitudinal direction as indicated by two-dot chain line arrows. Moreover, flows of the cooling water from both sides collide near the central part CP in the longitudinal direction, and rebounding of the cooling water easily occurs. Thus, in particular, a hot spot HS, to which the cooling water is not sufficiently supplied, is generated at the center of the lower end of the cooling water channel. As a result, a problem arises in that the temperature distribution in the electricity-generating surface becomes nonuniform.